The 36-year-old suspect was arrested early on Thursday in an operation involving police and Poland's Internal Security Agency (ABW), Interior Minister Marcin Kierwiński told reporters.
"Police have detained a person suspected of the murder of a Russian citizen, a killing that shocked the Polish public," Kierwiński said.
The suspect was carrying a Georgian passport and is believed to have links to organised crime, according to Kierwiński.
Authorities are also examining whether he may have been connected to other crimes committed in Poland, including offences dating back to 2022.
Kierwiński described the detention as a major success for law enforcement agencies and said a special investigative team had been working on the case since the killing occurred.
Two Belarusian citizens were initially detained in connection with the investigation but were later released, he added.
Tomasz Siemoniak, the government minister overseeing the country's security services, said investigators would closely examine the possibility that foreign intelligence agencies played a role in the killing.
He noted that foreign security services have previously been accused of recruiting criminals to carry out attacks abroad.
"We are taking such a scenario very seriously," Siemoniak said. "Police, the Internal Security Agency and prosecutors will intensively investigate why this happened and whether others were involved."
The victim, identified by prosecutors as 44-year-old Russian citizen Robert K., was publicly known as Semyon Skrepetsky, also known as Simon Skrepetski. Media reports described him as a dissident artist and an outspoken critic of Russian President Vladimir Putin.
Siemoniak said Polish security services had repeatedly warned the victim that he could be at risk and had offered him protection while he was in Poland.
"Unfortunately, he did not take advantage of that offer," Siemoniak said.
Authorities said the suspect was identified through the analysis of surveillance footage, communications data and witness testimony. After investigators determined his whereabouts, police launched an operation to apprehend him.
Siemoniak said the suspect is believed to have been "directly involved in carrying out the attack."
Prime Minister Donald Tusk earlier announced the detention on social media, saying authorities were also working to determine who may have ordered the killing.
Police spokesman Andrzej Fijołek said investigators believe the suspect acted alone in carrying out the shooting, though they are still examining whether anyone assisted in planning the attack or helping the suspect escape afterward.
The murder weapon has not yet been found, Polish state news agency PAP reported.
According to investigators, the killing took place on Monday morning in a residential area of Biała Podlaska, a city near Poland's border with Belarus.
A gunman approached the victim on a sidewalk and fired two shots from a handgun. After the victim fell to the ground, the attacker moved closer and fired three additional shots at close range.
The scene of the fatal shooting in the eastern Polish city of Biała Podlaska on Monday, June 15. Photo: PAP/Wojtek Jargiło
A postmortem examination found seven gunshot wounds, including injuries to the head, chest and back..
In remarks on Wednesday, the Polish prime minister said all indications suggested the killing may have been politically motivated, though investigators still needed evidence to support such a conclusion.
"If it turns out that this was a murder ordered by Russia, it would be a very serious matter with international implications," Tusk said. "That would be state terrorism," he added.
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Source: TVP Info, IAR, PAP